The present invention to a method and apparatus for calcining solid particulate material such as limestone and dolomite having a size of minus 1/4 inch or minus 3/8 inch. More particularly, the method and apparatus relate to calcining such materials without separating the ultra fine materials from the more coarse particles.
Prior to the present invention, it was known to calcine limestone in various furnaces such as a rotary kiln and to calcine limestone, dolomite and other materials having a size on the order of 1/4 inch to 3/8 inch in a furnace such as a fluidized bed reactor. Fine stone which is calcined in a fluidized bed reactor often consists of minus 1/4 inch or minus 3/8 inch material which means that a large percentage of the material is ultra fine on the order of minus 20 mesh. In a fluidized bed furnace, combustion air is passed upwardly through a bed of the material to be treated to maintain a fluidized bed. In order to maintain the material in a fluidized state, the fluidizing air must be supplied at a certain velocity. This velocity is such that the ultra fine material will be elutriated out of the fluidized bed reactor with the spent fluidizing gases prior to being calcined. Simply recycling the fines to the reactor will result in a repetition of the elutriation.
In order to overcome the problem of the ultra fines not being calcined, these fines can either be wasted which means wasting a valuable product, screened out of the feed material for separate treatment, or treated as in U.S. Pat. No. 3,588,063 wherein the fine material elutriated out of the fluid bed reactor is compacted and recycled to the reactor with new feed. None of these solutions is satisfactory as each requires unnecessary equipment or waste of material.
Prior to the present invention, it was known to calcine fine material on the order of 20 mesh in a flash calciner. A flash calciner for limestone is shown in U.S. Pat. No. 2,505,617. Although such fine material can be treated in a flash furnace, larger particles of material such as those normally treated in a fluidized bed reactor cannot be successfully calcined in a flash calciner.
Prior to the present invention, it is believed there is no known means for continuously treating all fine material of minus 1/4 inch size. The treatment of such material must be interrupted when considering the ultrafine material or dust which, by definition, will be a substantial part of minus 1/4 inch material.